vpx_dsp/x86/avg_intrin_sse2: fix int sanitizer warnings
vpx_dsp/x86/avg_intrin_sse2: fix int sanitizer warnings

Unit Test: VP9/AqSegmentTest. VP9/CpuSpeedTest, AVX2/Loop8Test6Param

implicit conversion from type 'int' of value 59741 (32-bit, signed) to
type 'int16_t' (aka 'short') changed the value to -5795 (16-bit, signed)

BUG=webm:1615

Change-Id: I2e5b688a97c3caa29d4b8a817b95a4986b81a562

webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx